The present invention relates generally to communication systems. The present invention more particularly relates to a cable modem system wherein information is communicated between a plurality of cable modems and a cable modem termination system that includes a plurality of cable modem termination system devices.
The desired solution for high speed data communications appears to be cable modem. Cable modem is capable of providing high data throughput rates, and is thus suitable for high speed file transfer, video teleconferencing and pay-per-view television. Further, cable modems may simultaneously provide high speed Internet access, digital television (such as pay-per-view) and digital telephony.
Although cable modems are used in a shared access system, wherein a plurality of subscribers compete for bandwidth over a common coaxial cable, any undesirable reduction in actual data rate is easily controlled simply by limiting the number of shared users on each system. In this manner, each user is assured of a sufficient data rate to provide uninterrupted video teleconferencing or pay-per-view television, for example.
Cable modem systems typically include one or more head ends or cable modem termination system (CMTS) devices that engage in bidirectional communication with the various subscribers"" cable modems. Both the cable modems and the CMTS devices include modulators to transmit data (either upstream from the cable modems to the CMTS devices, or downstream from the CMTS devices to the cable modems), as well as demodulators to receive and demodulate the incoming data. Such system are preferably flexible to accommodate varying numbers of subscribers (typically an ever-increasing number).
MAP information is transmitted on one or more downstream channels by the cable modem termination system to all of the cable modems on a given frequency channel. As is well known in the art, MAP information covers all time periods on an upstream channel. MAP information typically consists of the combination of one or more of the following: request regions (i.e., the contention area that a modem can request new band width), request/data regions (where both data and request can be transmitted), initial maintenance regions (where new modems have the right to try and sign on), station maintenance regions (for modems that are in operation), and short and long data grant regions (for transmitting data). The short and long data grants may either be based on a request or can also be unsolicited grants. The MAP will consist of a combination of these regions, all as decided by the MAP generator.
The present invention specifically addresses and alleviates certain deficiencies associated with the above-mentioned prior art.
According to an aspect of the invention, a plurality of CMTS devices are linked together to form a larger medium access control (MAC) domain. The CMTS devices are preferably synchronized to facilitate communication between the CMTS devices and the cable modems.
In another embodiment of the invention, MAP information is transmitted to one or more of the CMTS devices, with such MAP information then being passed on to the downstream cable modems. The MAP information is then transmitted to the rest of the CMTS devices of the system. Each of the upstream channels is uniquely identified so that each of the CMTS devices extracts only the relevant MAP information from the broadcasted information.
Thus, in one illustrative embodiment of the invention, a plurality of CMTS devices are linked together and synchronized to facilitate communication between the respective CMTS devices and the downstream cable modems. According to the invention, one of the CMTS devices is designated as a master device, and the other CMTS devices are designated as slave devices. The respective CMTS devices are connected to each other by means of a synchronization bus. A future time stamp value is generated based on the counter value of the master CMTS device, and the future time stamp value is broadcast over the bus and is received by the respective CMTS devices. When the time stamp counter in the master CMTS device reaches the generated future time stamp value, a control signal from the master CMTS device is broadcast over the synchronization bus. The slave CMTS devices then retrieve the future time stamp value and reset their respective local time stamp counters to the future time stamp value. In this manner, the CMTS devices are synchronized.
In another illustrative embodiment, MAP information is generated and transmitted to at least one CMTS device, which forwards it on to the cable modems. The MAP information is then transmitted to the other CMTS devices. Each CMTS device receives the MAP information and filters out the information that is irrelevant to that particular CMTS device. Each CMTS device determines the relevant information based on unique identifiers assigned to the respective upstream channels, which are included in the MAP information.